Abstract
Salmonella enterica is a Gram-negative, food-borne pathogen, which colonizes the intestinal tract and invades enterocytes. Invasion of polarized cells depends on the SPI1-encoded type III secretion system (T3SS) and the SPI4-encoded type I secretion system (T1SS). The substrate of this T1SS is the non-fimbrial giant adhesin SiiE. With a size of 595 kDa, SiiE is the largest protein of the Salmonella proteome and consists of 53 repetitive bacterial immunoglobulin (BIg) domains, each containing several conserved residues. As known for other T1SS substrates, such as E. coli HlyA, Ca2+ ions bound by conserved D residues within the BIg domains stabilize the protein and facilitate secretion. The adhesin SiiE mediates the first contact to the host cell and thereby positions the SPI1-T3SS to initiate the translocation of a cocktail of effector proteins. This leads to actin remodeling, membrane ruffle formation and bacterial internalization. SiiE binds to host cell apical membranes in a lectin-like manner. GlcNAc and α2–3 linked sialic acid-containing structures are ligands of SiiE. Since SiiE shows repetitive domain architecture, we propose a zipper-like binding mediated by each individual BIg domain. In this review, we discuss the characteristics of the SPI4-T1SS and the giant adhesin SiiE.
Highlights
Salmonella Enterica PathogenicitySalmonella enterica is a food-borne pathogen that is able to infect a broad range of hosts and colonizes various niches in infected hosts [1]
SPI2 encodes for a T3SS, but this system is responsible for the intracellular survival and proliferation of Salmonella inside host cells
Rather in line with the controlled SiiE retention by the putative proton channel formed by SiiA and SiiB, increase of the proton-motive force (PMF) by external acidification leads to a higher surface retention of SiiE, whereas PMF destruction by the uncoupler CCCP leads to a dramatic decrease of SiiE surface retention [45]
Summary
Salmonella enterica is a food-borne pathogen that is able to infect a broad range of hosts and colonizes various niches in infected hosts [1]. Functions are required for invasion of non-phagocytic cells by Salmonella. The SPI1-encoded T3SS translocates into host cells a cocktail of effector proteins leading to actin remodeling, membrane ruffling and uptake of Salmonella. SPI2 encodes for a T3SS, but this system is responsible for the intracellular survival and proliferation of Salmonella inside host cells (reviewed in [1]). Besides maintaining the SCV, SPI2-T3SS effector proteins are responsible for induction of tubular membrane vesicles, termed. To establish intimate contact to, and subsequently invade polarized epithelial cells, Salmonella requires the function of the SPI4-encoded type I secretion system (T1SS) and its substrate. The C-terminal moiety of SiiE mediates the first contact to the host cell apical membrane and this function may allow the proper positioning of the SPI1-encoded T3SS. This review focuses on structure, function and binding properties of this outstanding protein that is involved in Salmonella adhesion to and invasion of polarized epithelial cells
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